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Height Accuracy Reaches New Low • Terrestrial height network (leveling) is significantly compromised • Last systematic leveling in southern Alaska in 1964-1965, immediately after 1964 Prince William Sound earthquake • There has been up to 1.25 METERS of vertical crustal motion since then. • Substantial areas of >20-30 cm deformation since last survey • Geoid height errors in Alaska can be very large, as is well-known at NGS • GRACE geoid compared to EGM-96 showed up to 1 meter geoid height errors • CONCLUSION: Alaska needs an essentially new vertical reference network – just like starting over

Main Sources of Vertical Motion • Glacial-Isostatic Adjustment (“post-glacial rebound”) up to 3.5 cm per year • 1 meter change every 30-50 years • But signal is strongest in parts of Alaska that had minimal height information aside from tide level • Postseismic Deformation Following the 1964 earthquake up to 1.25 meters since 1964 • Other faulting-related deformation much smaller (several mm/yr) • Deformation assoicated with 2002 Denali fault earthquake substantial but localized • Rarely more than 10 cm vertical change away from fault • Postseismic changes continue, may reach 10 cm level

What Needs to be Done • Existing vertical datum is in error by >30 cm in much of southern Alaska, as much as 1 m in places. • Because of crustal movement since surveys that defined it. • May be difficult to even maintain a consistent definition with past • Repeating all the leveling effectively impossible • All southern coastal tide gauges have moved >30 cm since surveys used to define NAVD88 • Need new definition of vertical datum based on GPS • CORS plus some number of monuments in the ground • Need new geoid model • If Arctic Ocean can have a 5-10 cm geoid, why should (populated) Alaska have to settle for meter-level errors? • It’s almost like starting from scratch. • Might be better to start over with a new system based on ITRF. The Lower 48 will have to go that route some day.